This invention relates to the field of prosthetics, and more particularly, to an intervertebral disc prosthesis designed to replace a damaged intervertebral disc.
The human spine consists of twenty-four small bones known as vertebrae, or “vertebral bodies,” that protect the spinal cord and provide stability to the torso. The vertebrae are arranged in a column and stacked vertically upon each other. Between each vertebra is a fibrous bundle of tissue called an intervertebral disc. These intervertebral discs act as a cushion to the spinal column by absorbing energy and transmitting loads associated with everyday movement. They also prevent the vertebrae from rubbing against each other.
Each intervertebral disc comprises two distinct regions. A firm outer region, the annulus, maintains the shape of the intervertebral disc. An inner region, the nucleus, provides a resilient tissue that enables the disc to function as a shock absorber. Over time, the normal aging process causes the intervertebral discs to degenerate, diminishing their water content and thereby reducing their ability to properly absorb the impact associated with spinal movements. Diminished water content in the intervertebral discs may also cause the vertebrae to move closer together. Tears and scar tissue can weaken the discs, resulting in injury. When the discs wear out or are otherwise injured, a condition known as degenerative disc disease results. With this condition, discs do not function normally and may cause pain and limit activity.
The condition of degenerative disc disease can potentially be relieved by a surgical procedure called artificial disc replacement. In this procedure, the damaged intervertebral disc is replaced by an intervertebral prosthetic device (i.e., an artificial disc).
Many artificial discs have been proposed in the past. A typical prior art artificial disc comprises two metal endplates, one endplate that faces a superior vertebra and one endplate that faces an inferior vertebra. A bearing surface is provided between the two metal endplates, allowing the endplates to rotate relative to one another and generally mimic the motion allowed by a natural disc.
Although current intervertebral disc prosthetic devices have enjoyed success, it would be beneficial to add additional desirable features to the prosthetic devices. For example, the large size of many current intervertebral prosthetic devices requires the devices to be inserted from an anterior approach. An anterior approach presents numerous challenges to the surgeon including the fact that the insertion site may be near the aorta and vena cava. Therefore, it would be advantageous to provide an intervertebral disc prosthesis that is relatively small and includes additional features to facilitate a posterior insertion.
Another advantage would be to provide a prosthetic device that more closely mimics the restricted movements offered by a natural disc. The structure of the human spine generally allows significant degrees of flexion/extension (and particularly flexion) but allows lesser degrees of axial rotation (torsion) and lateral bending. Therefore, it would be advantageous to provide an intervertebral prosthetic device operable to allow a significant degree of flexion/extension in a patient while only allowing restricted axial rotation and lateral bending.